This invention relates to a circuit for applying reading, programming and erasing voltages to the programming gates of individual memory cells of floating-gate-type, electrically-erasable-programmable-read-only-memory (EEPROM) arrays.
An EEPROM cell of the type used with the circuitry of this invention is described in our co-pending U.S. patent application Ser. No. 07/056,196,filed June 6, 1987 and also assigned to Texas Instruments Incorporated.
The EEPROM cell of the foregoing Application has a tunnelling area between the floating gate and the source and is programmed with a voltage Vpp of perhaps +12 to +16 volts applied between the control gate and the source, with the source at 0 volts and with the drain allowed to float or tied to a low or reference voltage source. The programming voltage is typically applied for a period of 10 milliseconds and produces a shift in voltage threshold of approximately 4.5 volts or more. The EEPROM cell is erased with a voltage of perhaps -10 to -12 volts applied to the control gate, with the source at perhaps +4 to +6 volts and with the drain allowed to float or tied to a low or reference voltage source. The erasing voltages are typically applied for a period of 10 milliseconds and produce a voltage threshold of approximately 1.0 volt. During reading operation of the EEPROM cell, a voltage of perhaps +3 volts is applied to the control gate with the source at 0 volts and the drain at a read voltage of perhaps 1.5 volts.
In the EEPROM device of the foregoing application, the various supply voltage sources are generated from the one approximately 5-volt external supply voltage using charge-pump capacitors or other type voltage generators located on the memory chip. Circuits for switching from one voltage to one other voltage are well-known and such circuits are used to switch the bitline voltage from one value to another value when changing from the programming mode of operation to the erasing mode of operation, for example. However, there is a need for a circuit that will allow the wordline voltage to be switched among at least three values, a positive programming voltage level, a negative erasing voltage level, and an intermediate sense voltage level.